It has already been known to prevent sound waves from propagating into the environment right at the site of their occurrence, if possible, so that the environment is not affected too strongly by those acoustic sound waves. In order to form quiet spaces, it is further known to prevent, as far as possible, the sound from penetrating into those spaces from outside. Sound absorbers, which most of the time comprise sound absorbing materials, i.e. so-called "insulating materials" serve this purpose. However, material consumption is relatively high, which not only affects the production costs, but also the disposal of such insulating materials.
From DE 92 15 132 U1, there is known a molded part for use in the engine compartment of motor vehicles, which absorbs air sound and consists of a foil layer and a porous insulating layer. The molded part consists of an open porous PU foam which is sealed off by a PU foil on all sides.
Moreover, there are known sound absorbers (DE-U-92 15 132, DE-C-3 039 651, 4 011 705, 4 317 828 and 4 334 984) which comprise sound absorbing molded parts made of closed cellular PP foam, PE fleece bonded with a binder, polymeric materials or the like; uncovered Helmholtz resonators have also been used.
A sound absorber for absorbing sound from a relatively large frequency spectrum has also been known from U.S. Pat. No. 3,439,774, for instance. Therein, two layers are spaced apart from each other by a honeycombed spacer and provision is made for that the layers comprise micropores. The micropores are to be dimensioned according to a particular selection rule: the porosity of the outer layer, which faces the incident sound, shall comprise a relatively high permeability, i.e. penetrability for sound waves, and the other layer, which faces away from the incident sound, is to comprise a relatively low sound permeability. Such layers consist of stainless steel having a pore size of 50 to 500 .mu.m, for instance.
Another problem, namely the muffling of body sound, i.e. the muffling of sound generating body portions, is also resolved in that muffling masses like those being spaced apart by spacers are applied onto the body which vibrates and therefore generates sound waves according to U.S. Pat. No. 3,087,571 and 3,087,573 as well as FRP 2 671 899, for instance.